Cunbin An

Impact of Interfacial Microstructure on Charge Carrier Transport in Solution‐Processed Conjugated Polymer Field‐Effect Transistors

The surface roughness of a dielectric is precisely tuned, allowing a fine control solely over the interfacial microstructure in a semicrystalline semiconductor polymer film without affecting the morphology in the upper layers. The interfacial microstructure is found to have only a minor impact on the transport originating from bypassing of interfacial defects by the charge carriers.

Alignment of Organic Semiconductor Microstripes by Two-Phase Dip-Coating

assembled by manipulating the nucleation and the subsequent growth kinetics of conjugated molecules. Nevertheless, it is still a challenge to kinetically tune the assembly of conjugated building blocks into well-ordered, defect-free fi lm structures by solution processing, especially for less soluble molecules. In this communication, we propose a novel solution-processing approach, termed as two-phase dip-coating, to assemble both n- and p-type organic small molecules into highly oriented ultrathin microstripes with the assistance of a surfactant. In this method, an organic semiconductor solution is drop-cast on the surface of an aqueous surfactant solution. Subsequently, dip-coating is carried out, resulting in the alignment of ultrathin microstripes. Three main experimental parameters including aging time of the organic semiconductor solution, pulling speed of the substrate and surfactant concentration in the aqueous solution are investigated, demonstrating signifi cant infl uences on the self-assembly of the molecules and thus on the formation of microstripes. Moreover, for the n- and p-type conjugated crystalline molecules used in this study, the resultant transistors based on ultrathin aligned microstripes exhibit mobilities for electrons up to 0.12 cm 2 V −1 s −1 and holes of 0.16 cm 2 V −1 s −1 , respectively. Such pronounced performance for ultrathin layers confi rms that the charge carrier transport of organic semiconductors is primarily attributed to the fi rst few monolayers near the dielectric. [ 11 ] This work demonstrates that two-phase dip-coating is a promising solution-processing approach to deposit and align ultrathin layers of conjugated semiconductors with limited solubility.

Controlling the Surface Organization of Conjugated Donor–Acceptor Polymers by their Aggregation in Solution

The aggregation of conjugated polymers is found to have a significant influence on the surface organization of deposited films. Difluorobenzothiadiazole-based polymers show a strong pre-aggregation in solution, but the addition of 1,2,4-trichlorobenzene efficiently reduces such aggregates, leading to the transition of the surface organization from edge- to face-on orientation in deposited films.

Dithieno[2,3- d ;2′,3′- d ]benzo[2,1- b ;3,4- b ‘]dithiophene: a novel building-block for a planar copolymer

A planar heteroacene building block, dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;3,4-b′]dithiophene (DTmBDT), is reported via a facile synthetic procedure. Single-crystal X-ray diffraction of Br2-DTmBDT reveals that dodecyl chains interdigitate, still enabling close π-stacking of 3.42 A. A very high molecular weight quasi-planar copolymer PDTmBDT-DPP exhibited a high hole mobility of 0.36 cm2 V−1 s−1 in preliminary studies of organic field-effect transistors.

Investigation of the structure-property relationship of thiadiazoloquinoxaline-based copolymer semiconductors via molecular engineering

Six thiadiazoloquinoxaline (TQ) based copolymers (P1–P6) have been synthesized using Stille coupling reaction upon varying donor moieties, substitution positions and architectures of polymer side chains. UV-vis-NIR absorption spectra indicated that all of these polymers exhibited low optical bandgaps from 0.96 to 0.75 eV. The electron affinities of these six polymers with values from −3.92 to −3.99 eV slightly changed due to the contribution of the same TQ core. However, the ionization potentials were tuned from −4.95 eV in P1 to −5.28 eV in P3 by introducing different donors. Comparing polymers P1–P4 with different donor parts, two dimensional wide-angle X-ray scattering measurements implied that P4 had a higher crystallinity because its coherence length (5.8 nm) was 2–3 times higher than those of P1–P3 (1.7–2.9 nm). This led to a best field-effect hole mobility of 0.1 cm2 V−1 s−1 for P4 among the four polymers. The polymers P4–P6 had identical molecular formulae of side chains but significant differences in device performance. In comparison with P4, the two linear alkyl chains were moved onto head to head positions of bithiophene in P5, resulting in a hole mobility of only 0.007 cm2 V−1 s−1. However, a pair of 2-decyl-tetradecyl alkyl chains was grafted onto thiophene units adjacent to the TQ core in P6, leading to the highest hole mobility up to 0.24 cm2 V−1 s−1 in this series of polymers.

Synthesis of a quinoidal dithieno[2,3-d;2′,3′-d]benzo[2,1-b;3,4-b′]-dithiophene based open-shell singlet biradicaloid

A fused heteroacene derivative, bis(dicyanomethylene)-end-capped-dithieno[2,3-d;2′,3′-d]benzo[2,1-b;3,4-b′]-dithiophene (4CN-DTmBDT) was synthesized. Its open-shell biradical character in the ground state is studied by a combination of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). Structural assignment was verified with single crystal X-ray diffraction analysis. A highly stable biradicaloid with a low energy gap and low LUMO level is achieved.

Cyclopentadithiophene–Benzothiadiazole Donor–Acceptor Polymers as Prototypical Semiconductors for High-Performance Field-Effect Transistors

Donor-acceptor (D-A) conjugated polymers are of great interest as organic semiconductors, because they offer a rational tailoring of the electronic properties by modification of the donor and acceptor units. Nowadays, D-A polymers exhibit field-effect mobilities on the order of 10-2-100 cm2 V-1 s-1, while several examples showed a mobility over 10 cm2 V-1 s-1. The development of cyclopentadithiophene-benzothiadiazole (CDT-BTZ) copolymers one decade ago represents an important step toward high-performance organic semiconductors for field-effect transistors. The significant rise in field-effect mobility of CDT-BTZ in comparison to the existing D-A polymers at that time opened the door to a new research field with a large number of novel D-A systems. From this point, the device performance of CDT-BTZ was gradually improved by a systematic optimization of the synthesis and polymer structure as well as by an efficient solution processing into long-range ordered thin films. The key aspect was a comprehensive understanding of the relation between polymer structure and solid-state organization. Due to their fundamental role for the field of D-A polymers in general, this Account will for the first time explicitly focus on prototypical CDT-BTZ polymers, while other reviews provide an excellent general overview on D-A polymers. The first part of this Account discusses strategies for improving the charge carrier transport, focusing on chemical aspects. Improved synthesis as an essential stage toward high purity, and high molecular weight is a prerequisite for molecular order. The modification of substituents is a further crucial feature to tune the CDT-BTZ packing and self-assembly. Linear alkyl side chains facilitate intermolecular π-stacking interactions, while branched ones increase solubility and alter the polymer packing. Additional control over the supramolecular organization of CDT-BTZ polymers is introduced by alkenyl substituents via their cis-trans isomerization. The last discussed chemical concept is based on heteroatom variation within the CDT unit. The relationships found experimentally for CDT-BTZ between polymer chemical structure, solid-state organization, and charge carrier transport are explained by means of theoretical simulations. Besides the effects of molecular design, the second part of this Account discusses the processing conditions from solution. The film microstructure, defined as a mesoscopic domain organization, is critically affected by solution processing. Suitable processing techniques allow the formation of a long-range order and a uniaxial orientation of the CDT-BTZ chains, thus lowering the trapping density of grain boundaries for charge carriers. For instance, alignment of the CDT-BTZ polymer by dip-coating yields films with a pronounced structural and electrical anisotropy and favors a fast migration of charge carriers along the conjugated backbones in the deposition direction. By using film compression with the assistance of an ionic liquid, one even obtains CDT-BTZ films with a band-like transport and a transistor hole mobility of 10 cm2 V-1 s-1. This device performance is attributed to large domains in the compressed films being formed by CDT-BTZ with longer alkyl chains, which establish a fine balance between polymer interactions and growth kinetics during solvent evaporation. On the basis of the prototypical semiconductor CDT-BTZ, this Account provides general guidelines for achieving high-performance polymer transistors by taking into account the subtle balance of synthetic protocol, molecular design, and processing.

Effect of fluorination of naphthalene diimide–benzothiadiazole copolymers on ambipolar behavior in field-effect transistors

Two naphthalene diimide (NDI)–benzothiadiazole (BT) based conjugated polymers with high molecular weight, P1 and P2, were synthesized by introducing F atoms to modulate the electron-donating ability of the BT moiety. 3-Decyl-pentadecyl branched alkyl side chains were employed and expected to improve the molecular organization and device performance. Both polymers have excellent solubility in common organic solvents. UV-vis-NIR absorption and cyclic voltammetry indicate that the maximum absorption wavelength of P2 is blue-shifted and the HOMO energy level of P2 is decreased in comparison with P1. Two dimensional wide angle X-ray scattering of thin films revealed a similar organization of both polymers. A less balanced transport in field-effect transistors with increased electron mobility of 0.258 cm2 V−1 s−1 and lowered hole transport of 2.4 × 10−3 cm2 V−1 s−1 was found for P2. Polymer devices of P1 exhibited a balanced ambipolar transport, with a hole mobility of 0.073 cm2 V−1 s−1 and electron mobility of 0.086 cm2 V−1 s−1.

Thiadiazoloquinoxaline-Fused Naphthalenediimides for n-Type Organic Field-Effect Transistors (OFETs)

Thiadiazoloquinoxaline-fused naphthalenediimides (TQ-f-NDIs) are designed and synthesized. They show high electron affinities (EAs) of ∼4.5 eV. Organic field-effect transistor (OFET) devices, fabricated by dip-coating, provided maximum high electron mobilities of 0.03 cm2/(V·s) with an on/off ratio of 2 × 105.

Transistors: Impact of Interfacial Microstructure on Charge Carrier Transport in Solution-Processed Conjugated Polymer Field-Effect Transistors (Adv. Mater. 11/2016)

The interfacial microstructure of a semicrystalline semiconductor polymer film is precisely modulated by the dielectric with surface roughness ranging from 0.15 to 0.39 nm, without affecting the morphology in the upper layers, as described by K. Müllen, W. Pisula, and co-workers. On page 2245, they demonstrate that the interfacial microstructure has only a minor impact on transistor performance because charge carriers can bypass the interfacial defects.